1. Field of the Invention
This invention relates to hydrodynamic rotary seals for bi-directional rotation that are used to retain a lubricant and exclude an environment. More specifically, this invention relates to cooperative features that improve seal lubrication in conditions such as high operating temperature, skew-resisting confinement, high differential pressure, high initial compression, adverse tolerance accumulation, circumferential compression, high modulus seal materials, thin viscosity lubricants, third body seal surface wear, and/or material swell (collectively referred to as “severe operating conditions”).
2. Description of the Related Art
Hydrodynamic seals used in down-hole oilfield tools are being challenged to operate at ever-greater temperatures and differential pressures. Such seals are installed with interference (i.e., compression) and establish sealing by blocking the leak path. For general examples of such seals, see FIG. 3 of U.S. Pat. No. 5,230,520, FIG. 4 of U.S. Pat. No. 6,315,302, and FIG. 6 of U.S. Pat. No. 6,382,634.
Upon installation in a compressed condition, hydrodynamic seals define a “footprint” representing the shape of the “dynamic sealing interface,” and the two terms are generally interchangeable. Examples of footprints are shown in FIG. 2 of assignee's U.S. Pat. No. 4,610,319 and FIG. 13 of assignee's U.S. Pat. No. 5,230,520.
Smaller seal cross-sections are desirable because shaft and housing wall thickness can be maximized. Miniaturization impacts seal lubrication, as described in U.S. Pat. Appl. Pub. US2007/0205563, paras. [0036]-[0039]. For a given dimensional compression, interfacial contact pressure increases as a seal cross-section is miniaturized. With radial seals, circumferential compression increases as diameter is miniaturized, increasing footprint spread and contact pressure.
The skew-induced wear mechanism described and illustrated in FIG. 3-27 of the Kalsi Seals Handbook, Rev. 1 is addressed with skew-resisting confinement of the seal, which increases interfacial contact pressure and footprint spread. The term “skew-resisting confinement,” as used herein, encompasses (1) constraint imposed by seal contact with fixed location gland walls as disclosed in U.S. Pat. No. 6,315,302, and (2) spring-loading through a moveable gland wall, as disclosed in FIG. 3-28 of the Kalsi Seals Handbook, Rev. 1.
Generally, the conventional wisdom regarding how such hydrodynamic seals lubricate has been described in U.S. Pat. No. 4,610,319 at col. 9, lines 6-22 and U.S. Pat. No. 5,230,520, col. 3, lines 30-53. In the '520 patent, FIG. 13 uses a curved arrow to illustrate the conventional wisdom that a normal velocity component VN urges the lubricant toward the environment. The emphasis on VN has caused undue focus on inlet efficiency over the years, and diverted attention from finding other potential lubrication factors. Such conventional wisdom of how these seals operate has been repeated in numerous other patents and commercial literature. See, for example, U.S. Pat. Nos. 5,678,829 (col. 4, lines 14-33), 5,738,358 (col. 2, lines 17-57), 5,873,576 (col. 2, lines 26-65), 6,036,192 (col. 2, lines 26-65), 6,120,036 (col. 2, lines 18-45), 6,227,547 (col. 11, lines 16-40), 6,315,302 (col. 10, lines 31-46), 6,334,619 (col. 1, line 57-col. 2, line 5), 6,382,634 (col. 11, lines 4-9), 6,685,194 (col. 4, lines 51-55), and 6,767,016 (col. 1, line 27-col. 2, line 16). Additionally, the conventional wisdom has been that the footprint wave height, per se, is important to lubrication. “Footprint wave height” as used herein refers to the difference in width between the widest and narrowest parts of the footprint.
The use of the aforementioned conventional wisdom (relating to VN and footprint wave height) is inadequate in designing highly effective hydrodynamic rotary seals for use in severe operating conditions. The use of the conventional wisdom in the design of seals for severe operating conditions has resulted in limited seal effectiveness.
Another bit of the conventional wisdom pertaining to hydrodynamic rotary seals is related to physical hydrodynamic inlet convergence. A general (and correct) tenant is that more gradual convergence produces more efficient in-pumping. U.S. Pat. Nos. 6,315,302, 6,382,634 and 6,685,194 teach the use of gradual convergences. Experience has shown that despite their inlet efficiency, such seals lubricate sub-optimally because their designs are based on the conventional wisdom pertaining to footprint wave height and VN.
U.S. Pat. No. 6,109,618 teaches the use of abrupt trailing edge geometries, that are unsuitable as hydrodynamic inlets, on seals suitable only for uni-directional rotation. This abrupt geometry is on the trailing edges of the waves, and is coupled with a very gently converging inlet geometry on the leading edges. Due to the high hydrodynamic leakage of such geometry, and the small reservoir size of downhole tools, downhole seals cannot employ such geometries.
The prior art seals are constructed from elastomers which suffer accelerated degradation at elevated temperature. For example, media resistance problems, gas permeation, swelling, compression set, and pressure related extrusion damage all become worse at elevated temperature. A bi-directional rotation seal that operates with less torque and produces less seal-generated heat would be desirable, in order to moderate such degradation.
Circumferential slippage of a seal with respect to its groove occurs more often with large diameter seals because the moment arms between the static and dynamic sealing interfaces are more nearly equal, and the static sealing interface has less mechanical advantage. Rotational slippage is particularly undesirable in large diameter seals because the slippage can vary around the circumference of the seal, causing undesirable localized stretching. It is also undesirable in seals exposed to high differential pressure because slippage can accelerate seal extrusion damage. Slippage is exacerbated by seal or shaft wear because such wear increases running torque. A bi-directional seal that has lower running torque and more resistance to wear is therefore desirable.
It is desirable to overcome the aforementioned limitations of prior art seals.